Most Nexgard side effects are mild and resolve within 24 to 72 hours. Vomiting, the most commonly reported reaction, is described in clinical trial data as “self-limiting and of short duration.” However, because the active ingredient stays in your dog’s system for weeks, some effects can linger longer, and a small number of dogs experience more serious neurological reactions that need veterinary attention.
Common Side Effects and How Long They Last
Vomiting is the side effect reported most often in Nexgard clinical trials. It typically shows up within the first day or two after your dog takes the chewable and usually passes on its own without treatment. Many owners also report soft stool, diarrhea, decreased appetite, or lethargy in the same short window. These digestive and energy-related reactions generally clear up within one to three days.
One reassuring finding from the clinical data: vomiting tended to decrease with subsequent doses. So if your dog gets mildly sick after their first Nexgard chewable, the reaction often becomes less pronounced with future monthly doses.
Why Effects Can Persist Longer Than Expected
Nexgard’s active ingredient, afoxolaner, has a relatively long half-life in dogs of about two weeks. That means it takes roughly two weeks for your dog’s body to clear half of the drug from the bloodstream, and several more weeks to eliminate it fully. In some breeds, particularly Collies, the half-life can stretch significantly longer, up to 47 days in studies using a standard dose.
This long persistence is what makes Nexgard effective as a once-monthly flea and tick treatment. But it also means that if your dog is sensitive to the ingredient, mild side effects like low energy or reduced appetite could take longer to fully resolve than you’d expect from a single chewable tablet. There’s no way to flush the drug from the system faster once it’s been absorbed.
Neurological Reactions to Watch For
The FDA has issued an alert about neurological side effects associated with isoxazoline products, the drug class that includes Nexgard. These reactions are uncommon but can include muscle tremors, loss of coordination (walking as if off-balance), and seizures. The FDA notes that seizures can occur even in dogs with no prior seizure history.
Neurological symptoms are different from the typical digestive upset. They can appear hours to days after dosing and, because of afoxolaner’s long half-life, may not resolve as quickly as vomiting or diarrhea. If your dog shows any tremoring, stumbling, or seizure activity after taking Nexgard, contact your veterinarian right away rather than waiting it out.
Despite these warnings, the FDA still considers isoxazoline products safe and effective for most dogs. The neurological events are reported in a small fraction of treated animals.
Overdose Symptoms
Nexgard has a wide margin of safety, meaning dogs can tolerate doses well above the recommended amount without life-threatening consequences. In overdose situations, vomiting may be the only visible sign. Rarely, neurological symptoms like loss of balance, muscle tremors, low body temperature, or dilated pupils can appear. If you suspect your dog ate multiple chewables or a dose meant for a much larger dog, call your vet or an animal poison control center promptly.
What to Do While Your Dog Recovers
For mild reactions like a single episode of vomiting or a day of low energy, supportive care at home is usually enough. Make sure fresh water is available, offer bland food if your dog seems interested, and let them rest. Most dogs bounce back to normal within a day or two.
If vomiting continues for more than 24 hours, your dog refuses food for a full day, or you notice anything neurological (tremors, unsteadiness, staring episodes, or a seizure), those warrant a call to your vet. Because the drug can’t be removed from the body once absorbed, your vet’s role in more serious cases is to manage symptoms and monitor your dog until the drug clears naturally over the following weeks.
If your dog has had a significant reaction, discuss alternative flea and tick prevention options with your vet before the next monthly dose is due. Several non-isoxazoline products are available that use different drug classes and may be better tolerated by sensitive dogs.